This invention relates to an assembly in a scanner or imager where the field of view of a light sensor and the field of illumination of a light source are coplanar and coextensive in any plane where the target is located.
In conventional mark-sense and imaging apparatus, the light source and light sensor are discrete and have optical paths which generally are not congruent in space. Such an arrangement provides a field of illumination that includes optical patterns of interest but which in general is larger than the light sensor's field of view (termed herein the sensor's region of sensitivity). The light sensor generates a signal that is specifically responsive to light from the region of sensitivity that is incident on the light sensor.
When the field of illumination is larger than the region of sensitivity, flood illumination is said to be present. In this circumstance the process of accurately interpreting the information content of the indicia being scanned is interfered with, as will now be explained.
In FIG. 3 there is illustrated a typical prior art device wherein LEDs 121,121 illuminate a field of illumination 124 on target 123. A region of sensitivity 122, being smaller than field of illumination 124, is entirely included within field of illumination 124. The region of sensitivity 122 is viewed by light sensor 127 through optics 125, and presents a signal to amplifier 129 in response to detected light.
Mark-sense detectors are typically employed in optical scanning devices, such as bar code scanners or readers, in which the output of the detector may be coupled to decoding circuitry. There are a number of known optical effects that can produce errors in the determination-of transitions between bars and spaces in a bar code symbol that is situated on a substrate, including the diffusion and laminate effects.
The diffusion effect can be understood with reference to FIG. 4a and 4b. It results from light, shown representatively as beams 126, that is incident outside the region of sensitivity 122 of a light sensor. Beams 126 enter the substrate 138 on which a pattern is located, and are then scattered internally within the substrate into the region of sensitivity. A fraction of the scattered light is thus ultimately returned to the light sensor via region of sensitivity 122 where it contributes to the received signal level. In scanning applications such as bar code scanning there is another consequence of the diffusion effect. In this application the region of sensitivity travels across dark bars that are separated by fields of light spaces. These spaces are referred to herein as white spaces; however those skilled in the art will understand that areas of high reflectivity are denoted. As the region of sensitivity 122 approaches a dark bar 139, as shown in FIG. 4b, the bar 139 will absorb more light than does a white space of similar dimension. Since some light 128 is absorbed, less is available to scatter into the approaching region of sensitivity. Thus the diffusion effect differs quantitatively when a bar is near the region of sensitivity than when it is not. This can be appreciated with reference to FIG. 5 which shows an analog waveform 130 generated by a bar code scanner utilizing flood illumination which has scanned bar code symbol 132. It will be apparent that the signal minima 137 corresponding to the wide dark bars are quite sharp, while the maxima 135 corresponding to the intervening white spaces are rounded. Furthermore the peaks 136 corresponding to the narrow white spaces between the narrow dark bars 134 are reduced in amplitude relative to maxima 135 corresponding to wide white spaces 135 between the wide dark bars 137. This appearance is predicted by the above discussion of the diffusion effect. If the narrow dark bars could not be resolved due to the modulation transfer function of the reader's optical system, the patterns would have been symmetrical about the maxima and minima. While the diffusion effect has been explained in connection with a bar code scanner, it influences non-scanning readers of optical patterns as well. In summary, the diffusion effect tends to reduce the apparent white level in the vicinity of dark bars while not affecting the black level.
Another undesired effect of flood illumination in certain imaging applications, such as bar code scanning, is the laminate effect, also known as the overlay effect. Practical bar code symbols often possess protective overlaminates. The laminate effect is caused by the light that is scattered from outside the region of sensitivity being totally internally reflected at the overlaminate-to-air interface onto the region of sensitivity. Reference may be made to FIG. 6 where the laminate effect is depicted diagrammatically. During the lamination process an adhesive flows over the bar code symbol substrate 140, expelling the air between laminate 141 and bar code symbol 142. As a result, the laminate 141 and bar code symbol 142 are in intimate contact and are essentially index matched. When the laminated bar code substrate is illuminated, as with the diffusion effect, some of the incident light rays, such as rays 226a, 226b falling outside the region of sensitivity 122 are scattered from the top surface of layer 142 into overlaminate 141 and are incident on surface 227 at an angle 144 that can exceed the critical angle for total internal reflection. Such light rays 226a, 226b are totally internally reflected into the region of sensitivity 122. A portion of this light is then scattered back toward the overlaminate 141, penetrates it, and finally arrives at the light sensor, where it contributes to the received signal level. As in the case of the diffusion effect, as the region of sensitivity passes over the bar code symbol, some of the incident light will be absorbed by dark bars adjacent to the region of sensitivity, thereby reducing the white space signal level. The magnitude of the laminate effect is a function of laminate thickness, the dimensions of the bar code symbol, reflectance of the bars and spaces, nominal diffusion length in the substrate, and refractive index. While laminate effects are sometimes difficult to quantify, the range of this effect can be quite large. The laminate and diffusion effects are known to degrade the performance of bar code scanners, other types of mark-sense detectors, and image detecting apparatus generally. A laminated bar code symbol typically exhibits both the diffusion and the laminate effects.
It will be recognized that an analogous argument can be made for the case where a small source of illumination is used in conjunction with a large region of sensitivity, so called flood sensitivity. A number of methods have been employed in the prior art that tend to bring the beams of light from the light source and those directed to the light sensor from the target into alignment, establishing a common operational region. The images of the light source and the defined region of sensitivity are generally not congruent throughout a relatively large depth of field. For this reason, all these systems are sensitive to the degrading effects of the diffusion and laminate effects over all or most of their operational depth of field.
For example, in U.S. Pat. No. 4,346,292 to Routt, Jr. et al, there is shown an optical scanner in which source and reflected light beams are coaxially aligned at the target. In this device the region of sensitivity is substantially congruent with the field of illumination only in the immediate neighborhood of the focal plane, and it cannot achieve a larger depth of field without flood illuminating the target.
U.S. Pat. No. 4,675,531 to Clark et al shows a scanner that has coaxial incident and reflective beams, achieved by a multi-surfaced lens arrangement. The beams, while having the same optical axis, could not define a light source image that is congruent to a region of sensitivity except in the focal plane. This device would not be suitable for applications requiring a larger depth of focus where flood illumination is undesirable.
In U.S. Pat. No. 4,816,659 to Bianco et al an apparatus is shown in which a bar code symbol is illuminated by a lamp, and a reflected beam received by a photosensor. The illumination and detection optical axes are not coaxially aligned in the plane of the bar code symbol.